A Point Mutation in the Hair Cell Nicotinic Receptor Alters Structure and Function of the

TARANDA J; BALLESTERO J; KATZ E; MAISON, S.F; LIBERMAN, M.C.; VETTER D; BOULTER J; FUCHS P; ELGOYHEN AB

Phoenix, AZ

Conferencia; 31st Midwinter Meeting, Association for Research in Otolaryngology; 2008

A nicotinic receptor (nAChR) composed of the alpha9 and alpha10 subunits mediates synaptic transmission between medial efferent olivocochlear (OC) fibers and hair cells. A mouse with targeted deletion of the alpha9 gene has demonstrated that this subunit is necessary for classic suppressive effects of OC activation on cochlear responses. To complement the knock-out, we have generated mice with a gain-of-function mutation in the alpha9 subunit by homologous recombination. A leucine for threonine change was introduced at position 9. (L9´T) of the second transmembrane domain of the alpha9 protein. Responses to acetylcholine (ACh) recorded in voltage-clamp from P 8-P13 inner hair cells of acutely excised organs of Corti of L9´T knock-in (ki) mice showed a significant increase in sensitivity to exogenously applied ACh (EC50, 63.9±8.3 µM, n=5, wt; 38.9±6.3 µM, n=6, ki). In addition, responses to ACh desensitized more slowly in the ki (I30/Imax 25±3%, n=12, wt; 53±7%, n=8, ki). Spontaneous and electrically evoked postsynaptic currents also decayed much more slowly in the ki (τ= 61.8±2.4 ms, wt and 241.9±16.7 ms, ki). Cochlear histology was normal, as assessed by light-microscopic evaluation of plastic embedded sections. However, synaptophysin immunostaining of the middle cochlear turn in adults revealed an increase in the mean number of efferent terminals per outer hair cell (2.9 ± 0.4, ki; 2.0±0.4, wt; p<0.02), with as many as 6 terminals found on ki outer hair cells The present results indicate that a point mutation in the alpha9 nAChR subunit leads to changes in postsynaptic response and presynaptic structure of contacts between medial efferent neurons and cochlear hair cells.µM, n=5, wt; 38.9±6.3 µM, n=6, ki). In addition, responses to ACh desensitized more slowly in the ki (I30/Imax 25±3%, n=12, wt; 53±7%, n=8, ki). Spontaneous and electrically evoked postsynaptic currents also decayed much more slowly in the ki (τ= 61.8±2.4 ms, wt and 241.9±16.7 ms, ki). Cochlear histology was normal, as assessed by light-microscopic evaluation of plastic embedded sections. However, synaptophysin immunostaining of the middle cochlear turn in adults revealed an increase in the mean number of efferent terminals per outer hair cell (2.9 ± 0.4, ki; 2.0±0.4, wt; p<0.02), with as many as 6 terminals found on ki outer hair cells The present results indicate that a point mutation in the alpha9 nAChR subunit leads to changes in postsynaptic response and presynaptic structure of contacts between medial efferent neurons and cochlear hair cells.